1. Field of the Invention
The present invention relates to the field of fluorescent lamps supplied from the high voltage a.c. mains power system (for example, 220 volts/50 Hz or 115 volts/60 Hz). The present invention more specifically relates to the lamp control, essentially, in current limitation in nominal operation and in turn-on triggering.
2. Discussion of the Related Art
In nominal operation, it is necessary to provide in the lamp supply circuit a current limiting element due to the structure of fluorescent lamps. Indeed, this type of lamp behaves, in nominal operation, as a voltage limiting component, that is, the voltage across the lamp is independent from the supply voltage, and is determined by the power of the lamp itself. Accordingly, to supply a fluorescent lamp on the mains voltage, it is necessary to provide a current limiting component, generally called a "ballast".
At turn-on, it is necessary to provide a triggering or starting component, generally called a "starter", meant to, first, heat up the filaments of the lamp, then start the lamp with an overvoltage.
FIG. 1 shows an example of a diagram of a conventional fluorescent lamp power supply circuit. A lamp 1 is generally formed of a tubular piece T filled with gas and at both ends of which are provided two excitation filaments f, f'. Each filament is meant to be electrically connected by both its ends and is thus associated with two supply terminals 1, 2, respectively, 1', 2'. The two filaments f, f' are meant to be supplied by an a.c. voltage Vac, for example, the mains voltage applied between two supply terminals 3, 4, of the lamp circuit.
In the example shown in FIG. 1, the current limiting component is formed of a high value iron inductance L interposed between a first a.c. supply terminal 3 and a first terminal 1 of one of the filaments f of lamp T. The second terminal of filament f is connected, via a starting component 5, to a terminal 2' of the second filament f', the second terminal 1' of which is connected to the second mains supply terminal 4. A capacitor C interconnects terminals 3 and 4.
Triggering or starting element 5 is most often a thermal switch meant to heat up filaments f and f', of lamp T by short-circuiting terminals 2 and 2' as long as the filaments are cold. The thermal switch opens as soon as it has reached a given temperature, which causes an overvoltage which triggers the lamp by means of the power storage formed by inductance L.
The function of inductance L is, in nominal operation, to limit the current in lamp T so that its voltage does not exceed the value for which it is designed. The function of capacitor C is to compensate the dephasing associated with the inductive assembly in order to improve the power factor and to make it acceptable for a connection to the network.
A disadvantage of a conventional supply system such as shown in FIG. 1 is that the use of a high inductance (generally on the order of 1 Henry) results in a bulky and heavy system. Further, the inductive nature of the assembly which requires a compensation of the dephasing by capacitor C requires a capacitor of high value (generally of more than 10 .mu.F), which thus has to be an electrolytic capacitor.
Another disadvantage of such a system is that there exists no light dimmer for use with this system.
FIG. 2 shows an example of a conventional diagram of a so-called "electronic" limiting circuit, that is, a circuit using active components to limit the current of the fluorescent lamp in nominal operation.
Such a circuit is formed by a diode bridge 10, having two terminals that receive an a.c. voltage connected to two terminals 3, 4, that receive mains voltage Vac. A first rectifying output terminal 11 of bridge 10 forms a ground terminal of the circuit. A second rectifying output terminal 12 of bridge 10 provides, by means of a high value electrolytic capacitor C', a d.c. supply to a switched-mode converter 13 used to supply fluorescent lamp T. Switched-mode converter 13 generally is formed by a control circuit 14 associated with two MOS power transistors M1, M2 (or two bipolar transistors) connected in series between terminal 12 of bridge 10 and the ground, capacitor C' being connected in parallel to this series association. A terminal 15 of the switched-mode converter is connected to a first terminal of a high frequency inductance L' mounted, as in the case of FIG. 1, in series with one of the filaments f of lamp T. A capacitor C" of low value interconnects filaments f and f' and contributes to the lamp triggering. The second terminal 1' of filament f' is grounded via a capacitor 16. Another capacitor 17 connects terminal 1' to an input terminal 18 of switched-mode capacitor 13. Capacitors 16 and 17 are used to filter the d.c. component in lamp T. Terminal 18 receives the d.c. voltage provided by capacitor C'. Transistor M1 is connected between terminal 18 and terminal 15 and transistor M2 is connected between terminal 15 and the ground. Transistors M1 and M2 are controlled by circuit 14 which also includes a feedback input connected to terminal 15 and which is supplied from terminal 18 via a resistor R. A capacitor 19 interconnects terminals 15 and 18 and contributes to the generation of an auxiliary power supply necessary for the control of transistor M1.
Circuit 14 may include other configuration and parametering terminals, not shown. The operation of an electronic limiting circuit such as shown in FIG. 2 is perfectly well known. Bridge 10 and capacitor C' provide, for a 220 -volt a.c. voltage, a power supply on the order of 300 d.c. volts to the switched-mode converter which is of "symmetrical half-bridge" type. This converter provides an alternating current at a frequency which is generally approximately 30 kHz to fluorescent lamp T via the high frequency (series) inductance L', which may be of low value (on the order of one mH).
A system such as shown in FIG. 2 eliminates the use of a high inductance (L, FIG. 1).
However, a disadvantage of a circuit such as shown in FIG. 2 is that it still requires an electrolytic capacitor C' of high value (generally higher than 10 .mu.F) to filter the voltage rectified by bridge 10. The use of electrolytic capacitors may result in a reduced circuit lifetime.
Another disadvantage of the system shown in FIG. 2 is that it requires two high voltage MOS power transistors which operate at high frequency.
Another disadvantage of such a system is that it is required to add to bridge 10 a power factor correction circuit 20. Without circuit 20, the harmonics of the supply current strongly adversely affect the power factor.